Adding a Pinch of Salt: Using Ionic Mixed-Crystal Seeds to Search for New Solid Forms

Diverse crystalline forms of compounds of interest are sought in every field where solids are used, and better ways to find new forms are needed. The outcome of crystallizations can sometimes be directed by the presence of heteroseeds that do not have the same composition as the compound of interest but have a degree of structural similarity. When the targets of screening are salts, potential heteroseeds can be made by simple ionic substitution, rather than by complex multi-step syntheses. Mixed crystals produced by partial ionic substitution are especially attractive for use as seeds because they closely resemble the salt of interest in composition, but their lattices and surfaces are altered by the simultaneous presence of multiple components. We have tested the strategy of ionic mixed-crystal seeding by using it to seek new solid forms of paraquat dihalides, which are widely studied salts of commercial importance. Our results show that the strategy offers an especially convenient and effective way to discover new solid forms. Every thorough polymorphic screening of salts should include use of the method.